1. Field of the Invention
The present invention relates to an optical printer, and more specifically relates to an optical printer having a print head which scans the surface of a photosensitive member at constant speed via the ON/OFF switching of the light emitted by said print head in accordance with image information so as to form an electrostatic latent image on said surface of the photosensitive member, develops said electrostatic latent image, and thereafter transfers said developed image.
2. Description of the Related Art
Conventional optical printers using electrophotographic methods are well known wherein, as shown in FIG. 1, individual elements L1, L2, L3, L4 (the number of elements is actually greater) are switched ON and OFF in accordance with image information and as an LED optical head 1 moves at uniform speed in the arrow "a" direction parallel to the axis of the center of rotation of the photosensitive drum 2, and an electrostatic latent image is formed on the surface of the photosensitive drum 2 via the simultaneous rotation of the said photosensitive drum 2 in the arrow "b" direction.
The aforesaid type of optical printer has a disadvantage, however, inasmuch as the electrostatic latent image formed on the surface of the photosensitive member 2 is distorted at an angle .theta., as shown in FIG. 2, because the scanning speed of the optical head 1 is limited. When the scanning speed of the optical head 1 is designated Va, the peripheral speed of the photosensitive drum 2 is designated Vb, and the scan length is designated L, the amount of distortion Lp can be expressed by Equation (1) below. EQU Lp=L(Vb/Va) (1)
Furthermore, the distortion angle .theta. can be expressed by Equation (2) below. EQU .theta.=tan.sup.-1 (Lp/L)=Lp/L (2)
In FIG. 2, L1'-L4' indicates the optical irradiation positions corresponding to the individual elements L1-L4 of the optical head 1.
As shown in FIG. 2, when the electrostatic latent image having a distortion of angle .theta. is developed and transferred, the distortion is transferred directly to the transfer sheet. In order to eliminate this disadvantage, serial printers of the inkjet type and the like have been devised wherein the rotation of the photosensitive drum 2 is stopped during one scan by the optical head 1. However, methods of this type pose another disadvantage inasmuch as the drive systems are much noisier and transfer sheet transport control and rotation control of the photosensitive drum 2 are more complex.
Japanese Laid-Open Patent Application No. Hei 4-114181 discloses a method preventing image distortion by providing a twist to the scanning direction of the optical head 1 relative to the axial direction of the photosensitive drum 2. However, this method poses certain disadvantages in that the optical head scanning path must be constructed with complex curves (spirals) in order to prevent previously described image distortion, such that, when said scanning path approaches a straight line, the distance between said optical head and the surface of the photosensitive member cannot be maintained uniform throughout the entire scan region.